Well considering women with large breasts can suffer back pain because of the weight, I don't find this implausible...I can say that I am a DD and mine probably weigh a good 15-20 lbs. which would equate to one of my cats lying on someone's face.
I would think the guy would have bitten her or something, unless he was already unconscious and she just decided to finish him off that way.

Hard to confiscate the murder weapon though...
Would she be given her own cell so she doesn't try it on someone else?